The present invention relates generally to lighting ballasts for powering fluorescent lamps. More particularly, the present invention relates to rapid start or programmed start ballasts configured to reduce filament drive and to simultaneously balance pin currents through series connected filaments of series connected lamps.
A conventional lighting ballast design is represented in FIG. 3 and is further described herein as exemplary of problems currently existing in the art. Depending on the specific orientation of the ballast output lead wires, the steady state current flowing through the pins leading to the cathodes can exceed established manufacturer limits and significantly shorten the useful life of T4 and T5 fluorescent lamps. Ballasts designed with filament drive, commonly referred to as rapid start ballasts or programmed start ballasts, drive a common mode arc current (I_arc) through the length of the tube that is delivered via two pins on each end of the lamp (e.g., R_lamp1_pin1, R_lamp1_pin2 on a first end of Lamp1), and drive a differential mode current through both pins of one end of a lamp to heat the filament.
Current phasing and general imbalance of resistive elements used as part of the driving filaments can cause instantaneous current amplitudes to add and thereby increase the pin current. Series-connected filaments are at the most risk due to using a wire strap WS from the pin of one lamp (e.g., Pin 4 of Lamp 1) to the pin of the other lamp (e.g., Pin 2 of Lamp 2), bypassing any elements otherwise available to control the pin current. In this case the current through one pin can be high, most likely the pin connected to the wire strap WS, and the current through the other pin will be low.
It would therefore be desirable to reduce the quantity of energy driven into the filaments, particularly series-connected filaments, and better balance the current through pins associated with series connected filaments.
There are numerous approaches conventionally known in the art to indirectly accomplish these tasks. The majority of these approaches merely reduce the filament drive current by either controlling a separate filament drive system or by resonance means. These approaches are adequate if rebuilding the inverter and ballast tank are an option, but this of course involves significant design effort and has substantial effect on the overall ballast design when added to an existing topology.
There are also known methods specifically chosen to directly shunt excess filament drive current and arc current without also further reducing filament drive current.
One method in particular (as represented for example in FIG. 4) has a single inductor across the filament load. Depending on the operating frequency of the inverter and the value of the inductor and the filament drive capacitor, the filament drive can be amplified as a result of series resonance between the inductor and the filament drive capacitor. However, the amplification can dramatically increase the filament drive current, thereby increasing the pin current.